Certain physical processes involving laser irradiation require radiation intensities which grow in peculiar fashion, often faster than exponential, and over time periods substantially longer than the rise time of a single pulse (.about.25 picoseconds). One method of achieving this growth, known as pulse stacking, splits a single laser pulse into a sequence of time-delayed pulse images, changes the intensity of each pulse image, recombines the images to form a pulse envelope of desired shape. As few as seven pulse images often suffice for adequate pulse envelope definition.
One simple and successful method is the passive pulse stacking scheme disclosed in U.S. Pat. No. 3,879,109 to C. F. Thomas (issued Apr. 22, 1975) which uses two sets of partially transmissive parallel mirrors, spaced apart and with differing reflectivities, to obtain an exponentially increasing pulse envelope. The Thomas system is passive in that no physically active mechanism is required to produce the pulse sequence. This is at once both an advantage, in that it requires little or no maintenance and offers reproducible results, and a disadvantage, in that any change in system parameters usually requires replacement of one or more mirrors and realignment of all. Further, the temporal rate of growth of the pulse envelope is theoretically limited, as will appear below.
U.S. Pat. No. 3,675,022 to M. A. Nelson et al. (issued July 4, 1972) discloses the use of a Kerr cell system, with phase shift proportional to the square of the applied voltage, for modulating a laser beam, utilizing a birefringent liquid in the Kerr cell. Nelson et al. were concerned primarily with phase matching the optical (laser) signal and the electrical (applied voltage) signal as the signals simultaneously propagate through the Kerr cell.
U.S. Pat. No. 3,408,593 to H. Hurwitz (issued Apr. 30, 1964) discloses the use of a Kerr cell or a Pockels cell plus two crossed polarizers within the laser cavity (located between two or more reflective means) to act as a shutter of controlled duration for a pulse of polarized light issuing from the laser. The possibility of pulse shaping or of generating a series of modulated pulses is not considered by Hurwitz.
U.S. Pat. Nos. 3,297,876 and 3,518,436 to A. J. De Maria et al. (issued Jan. 10, 1964, and June 30, 1970, respectively) teach the use of Kerr cell plus two crossed polarizers, located either inside or outside the cavity of a mode-locked, Q-switched laser, to generate a sequence of short (10.sup.-3 - 10.sup.-9 second) duration, equispaced pulses. Again, the possibility of shaping the envelope of pulses by particular choices of Kerr cell voltage is not considered by De Maria et al.
Selection of a single picosecond laser pulse is obtained by the invention disclosed in U.S. Pat. No. 3,519,328 to D. D. Grossman (issued July 7, 1970) which uses a Kerr cell, two crossed polarizers and a chamber filled with an easily ionizable gas such as argon, the argon being triggered by the arrival of a portion of the original laser pulse. Here, the concern is with selecting a single pulse by short duration activation of the Kerr cell.
U.S. Pat. No. 3,532,890 to R. T. Denton (issued Oct. 6, 1970) discloses an optical multiplexing and demultiplexing system wherein a train of pulses is divided into a plurality of distinct pulse trains, appropriately delayed in time, and each pulse train is separately polarized, modulated, and analyzed using an electro-optical or magneto-optical material; the separate pulse trains are then interleaved again at the demultiplexing stage. Denton does not consider the possibility of temporally varying the electromagnetic modulation field; and, indeed, this might be detrimental for his purposes.
The above mentioned patents disclose the use of electro-optical modulators for control of the duration of the light pulse. As noted, none of the patent discloses contemplates use of such a modulator for temporal pulse shaping. No patents have been discovered which disclose use of a magneto-optical modulator for purposes of control of light pulse duration for temporal pulse stacking or shaping. Further, the Thomas patent, supra, appears to be the only one yet issued which discloses use of a passive system (requiring two pairs of parallel partially reflective surfaces) for temporal pulse stacking. However, the use of a Faraday rotator (a magneto-optical device) as a radiation isolator, to protect the system against damage due to back-reflected radiation, is well known.